The present invention generally relates to wireless communication systems, and in particular to minimizing interference from high-data-rate users in the uplink.
Wireless communication systems are widely deployed, providing voice and data communication services to mobile users. As wireless communication technology advances, mobile users may send and receive a wide variety of data—such as audio, video, images, e-mail, web browser content, and the like—in addition to traditional voice communications. Many such data transfers require much higher bandwidth than does digitally encoded voice. For example, enhanced uplink (EUL) packet access has been introduced in Wideband Code Division Multiple Access (WCDMA) 3GPP Release 6. With EUL, a packet bit rate as high as 5.76 Mb/sec is possible on the uplink (data transmission from a mobile terminal to a base station).
The required received power level at the base station (and consequently the power level at which each mobile terminal must transmit its signal) is proportional to the transmission data rate. Typically, there will be a large number of voice users transmitting at a low data rate on the uplink, simultaneously with a small number of high-data-rate users. In this case, the low-data-rate signals are subject to severe interference from the higher-power, high-data-rate signals. Similarly, a high-data-rate user is subject to severe interference from another high-data-rate user.
Several approaches are known for reducing or eliminating this interference. U.S. patent application Ser. No. 11/112,578, METHOD AND APPARATUS FOR CANCELING INTERFERENCE FROM HIGH-POWER, HIGH DATA RATE SIGNALS, filed Apr. 22, 2005, assigned to the assignee of the present application and incorporated herein by reference in its entirety, discloses a successive interference cancellation scheme. High-data-rate signals are detected first. When a high-data-rate signal is correctly detected (as indicated by, e.g., CRC checks), the receiver regenerates the high-data-rate signal and removes it from the received signal. The low-data-rate signals can then be detected based on the reduced-interference version of the received signal.
The parent U.S. patent application to the present application, Ser. No. 11/276,069, REDUCED COMPLEXITY INTERFERENCE SUPPRESSION FOR WIRELESS COMMUNICATIONS, filed Feb. 13, 2005, assigned to the assignee of the present application and incorporated herein by reference in its entirety, discloses a variety of schemes for sharing statistical interference information among multiple users. In one or more embodiments, interference from high-data-rate signals is treated as colored noise, and suppressed in a whitening matched filter, such as a Generalized RAKE receiver or chip equalizer.
RAKE receivers are well known in the communication arts and find widespread use in CDMA systems, such as in IS-95, IS-2000 (cdma2000), and WCDMA wireless communication networks. The name derives from the rake-like appearance of such receivers, wherein multiple, parallel receiver fingers are used to receive multiple signal images in a received multipath signal. By coherently combining the finger outputs in a weighted RAKE combiner, the conventional RAKE receiver can use multipath reception to improve the Signal-to-Noise Ratio (SNR) of the received multipath signal. RAKE receivers model interference as white noise, and work best when the interference and noise to be suppressed is white. A Generalized RAKE (G-RAKE) receiver improves interference suppression performance over a conventional RAKE receiver under some conditions (such as colored interference/noise) by increasing the sophistication of combining weight generation.
In the G-RAKE architecture, the combining weight calculations consider correlations of one or more signal impairments across RAKE fingers. For example, a G-RAKE receiver may track high-data-rate signal interference correlations across those fingers. G-RAKE receivers also may include a comparatively larger number of fingers such that extra fingers may be positioned off of the signal path delays. By using knowledge of how selected signal impairments are correlated across fingers, the G-RAKE receiver can compensate the finger combining weights such that receiver interference suppression performance is improved.
Conventionally, G-RAKE combining weights can be formulated asw=Ru−1h  (1)where Ru is an impairment covariance matrix and h is the net response. The G-RAKE combining weight is within a scaling factor of the tap coefficients of a linear Minimum Mean-Square Error (MMSE) chip equalizer, w=aw′ where a is a positive scaling factor, w′ is the tap coefficients vector of the linear MMSE chip equalizer,w=Rd−1h  (2)and Rd is the received signal sample correlation. Thus,w=aRd−1h  (3)
Recognizing that the received signal sample correlation Rd is the same for all uplink G-RAKE receivers, the parent application discloses calculating Rd for the union of finger delays needed by the various G-RAKE receivers in the uplink. Thus, if two G-RAKE receivers have the same finger delay pair, or have finger delay pairs of the same differential delay, they can share the same estimate of the received signal sample correlation. This may reduce or eliminate the need to calculate correlations Rd for each receiver.
However, when there are a large number of a finger delay pairs according to the G-RAKE finger delays of the various receivers, estimating Rd becomes computationally demanding. Additionally, using Rd in G-RAKE combining weight formulation needs the additional scaling factor a to produce a maximum-likelihood (ML) symbol estimate. The computation of a consumes additional resources, such as Digital Signal Processor (DSP) computational cycles. Accordingly, the ability to efficiently estimate the impairment covariance matrix Ru—from which combining weights may be calculated directly according to equation (1)—with sufficient accuracy and rapidity stands as a primary challenge associated with implementation of the G-RAKE receiver.